This invention pertains generally to the field of telecommunications and more particularly to repeated carrier systems, such as pulse code modulation (PMC) systems using a transmission cable pair as the transmission medium. The repeaters for such systems are usually powered via the transmission cable pair from a central office where the terminal equipment is located. A simplex power loop is usually provided at the repeater since the simplex power loop is a most attractive method for providing power to a signal line repeater. For a discussion and an illustration of a simplex power loop refer to "Bipolar Repeater for PCM Signals" by J. S. Mayo, Bell System Technical Journal, Janauary 1962, pages 73-76.
A transmission cable pair (hereinafter referred to as a signal line) with exposure to AC power transmission lines is susceptible to induced longitudinal AC currents. These induced currents potentially can cause transmission impairment with the communication signals and interference with the DC power feed to the signal repeaters installed along the signal line. The effect of the induced AC currents upon the simplex power loop is to cause the DC current in the simplex power loop (hereinafter referred to as line current) to be amplitude modulated usually at the power line frequency of 60 Hz. The induced AC modulation causes the instantaneous line current to vary above and below some nominal level. When the instantaneous line current is above the minimum level necessary to power the repeater, the excess power is usually converted into heat primarily in the signal line by the action of the voltage control circuitry within the repeaters. However, when the instantaneous line current is below the minimum level necessary for operation of the repeater, the repeater is starved for power and fails to perform properly.
The reduction of interference to the DC line current has usually been treated on a case by case basis using various techniques. One approach to the problem cancels the induced current by adding an opposing AC signal into the simplex power loop circuit at the end office repeater. Another method increases the available current margin by increasing the DC line current so that the minimum DC line current is adequate even when modulated by the induced AC current. Still another method is to reduce the DC current requirements of the signal repeater, which also increases the available current margin.